A subid passes information from your network to the advertiser. Usually an affiliate network passes back their affiliate's id as a subid to the advertiser. The advertiser would then collect this info and can see how many leads affiliate CD1 produced, CD2 produced, and so on. In short, a subid is what the publisher passes to the advertiser.
A transactionid is what the advertiser passes back to the affiliate network via the pixel. It is unique to each transaction that occurs when the pixel fires. Examples of transactionids are: an order number when you purchase something (which is always unique), or an email address when you register for a dating site. Whatever variable it is, advertisers should always pass back a unique value to the affiliate network. In short, a transactionid is a unique value that the advertiser passes back to the publisher.
This two way data sharing allows for several things. Assuming that the technology is in place, a subid allows the advertiser to see how many leads came in through each subaffiliate. They can easily determine if one source is really good, or really bad. Additionally, if your tracking ever goes down, they are able to tell you how many leads were produced through each subid so you can credit your affiliates accordingly.
A transactionid is necessary in case some transactions are fraudulent. In this case, for example, the advertiser provides you with an order number which was fraudulent or cancelled. With this information, you can see which affiliate produced the transaction and properly credit the advertiser for it. Additionally, if you are capturing leads or sales that are not matching with the advertiser's stats, you can send them a lead report with each lead or sale detail so they can check their database and resolve any lead discrepancies. The more information you are able to collect via the pixel, the better equipped you will be for handling any possible issues in the future.
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